1. Field of the Invention
The present invention relates to a method of preparing a modified polycarbonate resin. The method includes condensation polymerization and spray-crystallization to decrease a mole fraction of arylcarbonate existing in a non-reacted diarylcarbonate, and in a terminated group of a reaction side product having a degree of polymerization of less than 3 produced through melt condensation, so that an increase of the molecular weight of the modified polycarbonate resin produced through solid polymerization can be maximized. In this method, spray crystallization method is used, and a separate milling and drying processes are not required. The spray crystallization and the solid polymerization sequentially or concurrently in one reactor occur without additional drying, milling and fractionation processed so that the time required to prepare a modified polycarbonate is substantially reduced.
2. Description of the Related Art
Polycarbonate resins have excellent heat resistance, impact resistance, mechanical strength, and transparency. Due to these advantages, polycarbonate resins are used in various applications, such as compact discs, transparent sheets, packaging materials, and ultraviolet (UV)-blocking films, and thus, the demand for polycarbonate resins is increasing.
However, polycarbonates have low solvent resistance, and low impact resistance at low temperature. For example, when polycarbonates are exposed to commonly used solvents, crazing or cracking occurs. Thus, many efforts have been made to overcome these problems, and various modified polycarbonates have been developed. The modified polycarbonates mean the polycarbonate having not only carbonate groups but also other functional groups, for example, ester groups, ether groups, sulfide groups, sulfoxide groups, and siloxane groups, etc., in their chemical structures. In particular, polycarbonates modified with siloxanes exhibit superior low-temperature impact resistance, and processibility.
Conventional manufacturing processes for producing modified polycarbonates may be categorized into interfacial polymerization process in which phosgene is used, and melt polymerization process and solid polymerization process in which phosgene is not used.
First of all, in the case of polycarbonate modified with ester group, we can find the followings.
An interfacial polymerization process disclosed in U.S. Pat. No. 4,983,706 includes reacting an aromatic dihydroxy compound, such as a bisphenol-A, with phosgene, and a dicarboxylic acid. However, the process is accompanied by a risk caused by the use of harmful chemical materials, phosgene, and a chloride-based organic solvent, which is an environmental pollutant, and thus, equipment cost and manufacturing cost are high.
U.S. Pat. No. 6,232,429 discloses the method, by which a modified polycarbonate can be produced through melt polymerization, after transesterification reaction between an aromatic dihydroxy compound and a carbonic acid diester and esterification reaction of an aromatic dihydroxy compound with a dicarboxylic acid compound, which are performed at the same time using an alkali earth metal catalyst and a quaternary ammonium catalyst. This melt polymerization process is relatively stable because a poisonous material is not used. However, in order to produce high-molecular weight modified polycarbonate for extrusion, the highly viscous reactants must be handled at a high temperature and in a high vacuum, and these conditions lead to a low quality of modified polycarbonate.
U.S. Pat. No. 6,365,702 discloses a solid polymerization method for a modified polycarbonate. More particularly, a polycarbonate oligomer, a carboxylic acid compound, and a diarylcarbonate compound are subjected to esterification and transesterification using an antimony-based catalyst, and the resultant product is cooled and milled. The milled product is subjected to the solid polymerization. In this method separate milling and drying processes should be required.
Further, in the case of polycarbonate modified with siloxane, we can find the followings.
U.S. Pat. No. 5,530,083 discloses an interfacial polmerization process including reacting an aromatic hydroxy compound, a dihydroxy compound and hydroxyaryl-terminated dioganopolysiloxane with a phosgene, by using a catalyst. In this case, a high-molecular weight polycarbonate modified with siloxane resin can be easily produced continuously. However, the process is risky due to the use of a poisonous gas and an environmentally harmful chloride-based organic solvent, and thus, equipment cost and manufacturing cost are high.
A melt polymerization process includes polymerizing a melted starting material, as disclosed in U.S. Pat. No. 6,252,013. The melt condensation polymerization is less risky because poisonous materials are not used. However, in order to produce a modified high-molecular weight polycarbonate for extrusion, highly viscous reactants must be handled at a high temperature and in a high vacuum, and these conditions lead to a low quality of modified polycarbonate.
A solid polymerization process is a process in which a low-molecular weight modified polycarbonate prepolymer is crystallized and the crystallized prepolymer is polymerized more at a temperature lower than a melting point of the crystallized prepolymer. In this case, a poisonous material is not used, and the quality of an obtained modified polycarbonate can be guaranteed because the polymerization occurs in a solid state. In general, however, the crystallization and the solid polymerization are performed without removing reaction side products having a degree of polymerization of less than 3 and non-reacted diarylcarbonate which co-exists with a relatively low-molecular weight prepolymer (weight average molecular weight of 2000 to 20000 g/mol.) As a result, the stoichiometric balance between hydroxyl group and arylcarbonate group is broken, and the production time for a high-molecular weight modified polycarbonate increases. In this method, the amorphous prepolymer is crystallized by dissolving it in a solvent and then precipitating it. Therefore, the size and the crystallinity of the crystallized prepolymer are irregular, and separate drying, milling and fractionation processes are needed.
Accordingly, more research needs to be carried out to develop a modified polycarbonate manufacturing method that is stable, guarantees quality, and requires a short time to produce a high-molecular weight modified polycarbonate.